1. Field of the Invention
The present invention relates to a zoom lens, which is suitable for an image pickup apparatus such as a digital still camera, a video camera, a film camera, or a TV camera.
2. Description of the Related Art
Recently, image pickup apparatuses (cameras) using solid-state image pickup devices, such as video cameras and digital still cameras, have become smaller with increased functionality. Optical systems used in these cameras have been demanded to be small-sized zoom lenses with high optical performance. In any of these cameras using solid-state image pickup devices, various kinds of optical components such as an optical low-pass filter and a color correction filter are arranged between a lens system and the image pickup device. Therefore, the optical system for use in such a camera requires a lens system with a relatively long back focal length.
For a color camera with an image pickup device for color images, an optical system with good telecentricity on the image side is desired to avoid color shading. As a zoom lens with a reduced total system size and a long back focus and good telecentricity on the image side, a negative-lead zoom lens in which a lens unit with a negative refractive power is provided as a leading unit (located nearest to the object side) has been known in the art.
As the negative-lead zoom lens, a zoom lens comprising three lens units (hereinafter, also referred to as a three-unit zoom lens) has been known in the art. The three-unit zoom lens includes a first lens unit having a negative refractive power, a second lens unit with a positive refractive power, and a third lens unit with a positive refractive power in order from the object side to the image side (see, for example, Japanese Patent Laid-Open No. 2003-131132, U.S. Publication No. 2008/0043341, and U.S. Pat. No. 7,289,275).
As one kind of the three-unit zoom lens, a small-sized three-unit zoom lens in which all of the lens units move while zooming to achieve a high zoom ratio has been known (see, for example, Japanese Patent Laid-Open No. 2004-061675 and U.S. Pat. No. 7,333,275).
For reducing a total system size while attaining a high zoom ratio in the negative-lead type three-unit zoom lens, it is effective to reduce the number of lenses in each lens unit of the zoom lens and enhance the refractive power of each lens unit.
However, for example, an increase of aberration variation occurs when zooming if the refractive power of each lens unit is simply enhanced to achieve a high zoom ratio. Thus, the zoom lens hardly obtains high optical performance in the entire zoom range.
Therefore, for reducing a total system size, extending an angle of view, and enhancing a zoom ratio in the negative-lead three-unit zoom lens, it is important to appropriately determine the configuration of lenses in each lens unit, the distribution of refractive power in each lens unit. It is also important to appropriately define the moving locus or the like of each lens unit at the time of zooming.
For example, it is difficult to obtain high optical performance while reducing a total system size and extending an angle of view if the configuration of lenses, the refractive power, or the like in the first lens unit are not defined appropriately. In addition, in a three-unit zoom lens employing a rear-focusing system where the third lens group moves when zooming and focusing, it is also important to appropriately define the refractive power, the configuration of lenses, or the like in the third lens unit.
If the configuration of lenses in each lens unit is not appropriately defined, aberrations vary more extensively when zooming and focusing. Thus, it becomes very difficult to obtain high optical performance with respect to the whole zooming range and the entire object distance.